The present invention relates to a process for freeing an aqueous dispersion of a polymer from a monomer present therein, the polymer, which may be a homopolymer, graft polymer or copolymer, having an extremely minor concentration of residual monomer after purification.
It has long been known that aqueous polymer dispersions can be freed from volatile constituents by flowing an inert gas or steam at about 60.degree.-70.degree. C. through the dispersion, i.e. by subjecting the dispersion to steam distillation. This has been described in German Published Specification ("Auslegeschrift") No. 1,248,943, and in "Kunststoffe" (1959), volume 49, No. 10, page 499, and also in "Chemical Engineering", March 1972, page 96.
In those cases in which aqueous polyvinyl chloride suspensions are worked up under the conditions just described, the polymer is subsequently dried by means of air, whereby it is freed from a further proportion of vinyl chloride, e.g. about 2 weight %, based on the quantity of vinyl chloride subjected to polymerization, which is allowed to escape into the atmosphere. In other words, the issuing gas contains vinyl chloride in proportions which are clearly beyond an acceptable emission limit, namely beyond the limit of 150 mg of vinyl chloride per cubic meter of issuing gas. In addition to this, excessive proportions of vinyl chloride go into the waste water. Despite this, the final dry polyvinyl chloride still contains several hundred ppm of monomeric vinyl chloride, which is absorbed in the polymer and cannot be removed therefrom by the purifying procedure described above.
One of the uses of polyvinyl chloride sheets is in packing food, which is hazardous inasmuch as residual monomers contained in the polymer may migrate into the food. It is therefore obligatory for the dry monomer-containing polymers to be subjected to an additional special purifying treatment.
A process wherein dry polyvinyl chloride made in conventional manner is freed from residual vinyl chloride, which is embedded or occluded in the polymer particles, has been described in German Published Specification ("Offenlegungsschrift") No. 2,331,895. This process, which enables polyvinyl chloride to be freed from vinyl chloride and further comonomers, if any, comprises: heating the polymer to a temperature ranging from its freezing temperature to 180.degree. C. by directly condensing steam thereonto; maintaining the polymer at that temperature for the period necessary to free it from the bulk of monomer or monomers therein; and cooling the polymer down to a temperature lower than its freezing point by evaporating the steam condensed on the polymer. A preferred embodiment of this process comprises heating the polymer to a temperature ranging from 80.degree. to 130.degree. C. and allowing it to remain at that temperature for a period of about 5 minutes up to 2 hours, especially 10 to 60 minutes. Typical of this known process is that the degasification is effected at the dew point of water, as described in the working Examples of that Specification.
A disadvantage encountered with this earlier process resides in the fact that the polymer so purified continues to present relatively high proportions of monomers. As shown in Example 1 of German Published Specification ("Offenlegungsschrift" No. 2,331,895, the purified polymer contains 3 g (or 3,000 ppm) of residual monomers per kg of polymer. This known process is effected at temperatures and under pressures which correspond to the dew point of water under the conditions selected, which naturally means high and commercially unattractive consumption of steam.